An inductive load current control circuit which is used in a switching power supply, an inverter for controlling a motor, or the like, alternately turns on two switch elements connected in series between an input voltage and a ground potential, and controls a time period during which one of the two switch elements is turned on, and thus controls a current (inductor current) flowing to an inductor (inductive load) connected to a connection point between the two switch elements. In recent years, a technique for accurately detecting the current flowing to an inductive load is required for a power supply such as a DC-DC converter, which controls the inductive load by using a switch element.
A step-down DC-DC converter which outputs a voltage lower than an input voltage will be described below. In step-down DC-DC converters, a converter called synchronous rectification type alternately turns on and off first and second switch elements that are connected in series between an input voltage and a ground potential, and thus a potential at a connection point between the two switch elements is alternately conducted to the input voltage and the ground potential. The voltage is then averaged by a low-pass filter having an inductor and a capacitor to output a DC voltage to an output terminal. An error voltage obtained by amplifying a difference voltage between the output voltage and the reference voltage is converted into a pulse-width modulation signal by a PWM converter, and a ratio (duty cycle) of times for alternately turning on/off the first and second switch elements is controlled, so that an output voltage is controlled to be a target value.
Further, in a technique in recent years, a control method of monitoring a current flowing to an inductor and switching on and off when the current reaches a predetermined current is employed. As the method of monitoring an inductor current, two methods are known. One of the methods is a method of monitoring the current flowing into the first switch element disposed closer to the input voltage of the first and second switch elements connected in series with each other between the input voltage and the ground potential, and controlling the maximum value of a triangle-shaped current flowing into the inductor. The other method is a method of monitoring a current flowing into the second switch element disposed closer to the ground potential of the first and second switch elements connected in series with each other between an input voltage and a ground potential, and controlling the minimum value of a triangle-shaped current flowing into the inductor.
It is known that when a step-down DC-DC converter is operated in a low duty cycle, a high-speed switching frequency may be applied more easily by controlling the minimum value of a current than by controlling the maximum value of the current (see JP-A-2001-136737, for example).
With reference to FIG. 5, the following describes a conventional step-down DC-DC converter disclosed in JP-A-2001-136737 which uses the method of controlling the minimum value of a triangle-shaped current flowing to an inductor.
FIG. 5 is a circuit diagram showing the configuration of a typical conventional step-down DC-DC converter (power supply). An input terminal 117 is connected to one terminal of an external power supply 104 which outputs a DC voltage. Another terminal of the external power supply 104 is connected to a ground terminal 118 connected to a ground potential. The conventional step-down DC-DC converter (power supply) shown in FIG. 5 inputs the DC voltage output from the external power supply 104 through the input terminal 117 and the ground terminal 118.
A first switch element (high-potential-side switch element) 119 and a second switch element (low-potential-side switch element) 120 are connected in series with each other between the input terminal 117 and the ground terminal 118. A source of the first switch element (high-potential-side switch element) 119 which is a p-channel FET is connected to the input terminal 117. A source of the second switch element (low-potential-side switch element) 120 which is an N-channel FET is connected to the ground terminal 118.
One terminal of an inductor 123 is connected to a connection point 122 between the drains of the high-potential-side switch element 119 and the switch element of the low-potential-side 120 and an inverted input terminal of a current detecting amplifier 501. Another terminal of the inductor 123 is connected to one terminal of a filter capacitor 124 and an output terminal 125.
An external load (not shown) is connected between the output terminal 125 of the step-down DC-DC converter and the ground terminal 118.
Two input terminals of the current detecting amplifier 501 are connected to both terminals of the switch element of the low-potential-side 120, respectively, and output voltages which are in proportion to the step-down voltage.
A reference voltage generating section 101 outputs a reference voltage VREF.
A non-inverted input terminal of an error amplifier 102 is connected to the reference voltage generating section 101 to input the reference voltage VREF. The inverted input terminal is connected to the output terminal 125 to input an output voltage VOUT. The error amplifier 102 outputs an error voltage obtained by amplifying a difference voltage between the reference voltage VREF and the output voltage VOUT to an error voltage input terminal 126.
A non-inverted input terminal of a comparator 502 is connected to the output terminal of the error amplifier 102 through the error voltage input terminal 126, and the inverted input terminal of the comparator 502 is connected to the output terminal of the current detecting amplifier 501. The comparator 502 compares a voltage which is proportional to the step-down voltage of the switch element of the low-potential-side 120 outputted from the current detecting amplifier 501 with an error voltage output from the error amplifier 102. When the step-down voltage of the switch element of the low-potential-side 120 is lower than the error voltage, the comparator 502 outputs High, and otherwise, the comparator 502 outputs Low.
An oscillator 115 outputs a clock of an operation frequency of the step-down DC-DC converter in FIG. 5.
A switch element control circuit 116 is a set/reset flip-flop of a leading edge trigger. A set terminal of the switch element control circuit 116 is connected to the comparator 502 to input an output voltage from the comparator 502. A reset terminal of the switch element control circuit 116 is connected to the oscillator 115 to input a clock output from the oscillator 115.
The switch element control circuit 116 which is an RS flip-flop, is set in a reset state when the clock input to the reset terminal is switched from Low to High. In the reset state, the switch element control circuit 116 sets the first switch element 119 in a cutoff state and sets the switch element 120 in a conductive state.
The switch element control circuit 116 is set in a set state when an output voltage from the comparator 502 input to the set terminal is switched from Low to High. In the set state, the switch element control circuit 116 turns on the first switch element 119 and turns off the second switch element 120.
In FIG. 5, the current detecting amplifier 501, the comparator 502, the oscillator 115, the switch element control circuit 116, the input terminal 117, the ground terminal 118, the first switch element 119, the second switch element 120, the inductor 123, the output terminal 125, and the error voltage input terminal 126 constitute a conventional inductive load current control circuit.
An operation of the step-down DC-DC converter using the conventional inductive load current control circuit having the above configuration will be described below. An external load (not shown) is connected between the output terminal 125 of the step-down DC-DC converter and the ground terminal 118.
The switch element control circuit 116 is set in a set state at a start-up. The switch element control circuit 116 turns on the first switch element 119 of the high-potential-side, and turns off the second switch element 120 of the low-potential-side. A current is supplied from the external power supply 104 to the filter capacitor 124 and the external load through the input terminal 117, the first switch element 119, and the inductor 123. An inductor current IL(t) increases while time t passes, and energy is accumulated in the inductor 123. When this state is continued, the inductor current continuously increases with time.
The switch element control circuit 116 inputs a clock output by the oscillator 115 from the reset terminal every predetermined time. The switch element control circuit 116 is set in a reset state when a clock input from the reset terminal is switched from Low to High. The switch element control circuit 116 turns off the first switch element 119 of the high-potential-side and turns on the second switch element 120 of the low-potential-side.
The inductor 123 has such characteristic that the inductor current continuously flows while holding a previous state by the energy accumulated in the inductor 123. The inductor current is supplied from the ground terminal 118 to the external load connected to the output terminal 125 through the switch element of the low-potential-side 120 and the inductor 123.
When the switch element of the low-potential-side 120 is turned on from off, a voltage which is output from the current detecting amplifier 501 and in proportion to the step-down voltage of the switch element of the low-potential-side 120 is higher than the error voltage output from the error amplifier 102. The comparator 502 outputs Low. In this state, the inductor current decreases with time.
When the step-down voltage of the second switch element 120 of the low-potential-side becomes lower than the error voltage, the output from the comparator 502 is switched from Low to High. Thus, the switch element control circuit 116 is set in a set state again to turn off the second switch element 120 of the low-potential-side and to turn on the first switch element 119 of the high-potential-side. A current is supplied from the external power supply 104 to the filter capacitor 124 and the external load through the input terminal 117, the first switch element 119, and the inductor 123. The inductor current IL(t) increases with time t, and energy is accumulated in the inductor 123.
The above operation is repeated. When the circuit is in an equilibrium operation state, with respect to the two input signals of the comparator 502, the minimum value of the triangular-shaped voltages output from the current detecting amplifier 501 is equal to the value of the error voltage output from the error amplifier 102.
Thus, the conventional step-down DC-DC converter (power supply) monitors a current flowing in the second switch element 120 of the low-potential-side to control the minimum value of triangular-shaped currents flowing to the inductor 123.
Patent Document 1: JP-A-2001-136737